Field of the Invention
The present invention relates to a passive Q-switch laser applied to a spectroscopy, a laser machining device and a laser lighting device, and an operation optimization method thereof.
Description of the Related Art
FIG. 4 is a diagram illustrating a structure of the conventional passive Q-switch laser (Patent Document 1, 2). Referring to FIG. 4, the passive Q-switch laser comprises an excitation source 1, lenses 2a, 2b, a mirror 5a, a laser medium (gain medium) 3, a saturable absorber 4 and a mirror 5b. An optical resonator comprises the mirror 5a, the laser medium 3, the saturable absorber 4, and the mirror 5b. 
The excitation source 1 comprises a laser diode for excitation (pumping) and outputs the excitation light, which is excited by the laser diode and has a wavelength approximately 808 nm, to a lens 2a. The lenses 2a, 2b converge the excitation light from the excitation source 1 and outputs to the laser medium 3.
The laser medium 3 that is in-place between the mirror 5a and the mirror 5b comprises Nd:YAG crystals which is excited by the light having approximately 808 nm wavelength and emits the laser beam having approximately 1064 nm wavelength when transiting from the high-energy state (excited level) to the low-energy state (ground level).
The mirror 5a that transmits the light having the wavelength approximately 808 nm and reflects the light having the wavelength approximately 1064 nm with a high-degree of reflection is mounted to one end of the laser medium 3. The mirror 5b partially transmits light having the wavelength 1064 nm and reflects the rest of the light.
The saturable absorber 4 that is in-place between the mirror 5a and the mirror 5b increases the light transmission in accordance with absorption of the laser beam from the laser medium 3. The saturable absorber 4 becomes transparent when the electron density in the excited level is saturated and the Q-value of the light resonator rapidly increases and laser oscillation takes place and the pulsed light is emitted.
In such case, a quasi-continuous-wave (QCW) using the repetition frequency is used to excite the laser to suppress heat caused by the laser medium 3. At this time, the repetition frequency of the output laser is the same as the excitation-repetition frequency.
When obtaining 355 nm UV-output, the original wave (pump wave) that is an output of the passive Q-switch laser is converted to the second harmonic by the second harmonic generation (SHG) 6. The original wave and the second harmonic is converted to the third harmonic by the third harmonic generation (THG) 7. In addition, a phase matching to convert the wave is carried out by adjusting the angle of the SHG 6 and the SHG 7 relative to the optic axis. Temperatures of the SHG 6 and THG 7 are controlled to adjust finely the phase matching.
In addition, the pulsed repetition frequency (hereafter repetition frequency) that is the same as the output frequency excites the laser diode to obtain the desired output frequency. The excitation power of the laser diode (pump power) is set to the maximum energy level that is ordinarily used and the pulse width is set to a bit longer than the threshold value of oscillation.
When the repetition frequency varies, the method to add an offset to the pump power of the laser diode is disclosed (Patent Document 3). Or the method to control the pump of the laser diode by performing feedback of the part of output to the pump controller is disclosed (Patent Document 4).